The practice of ellipsometry is well established as a non-destructive approach to determining characteristics of sample systems, and can be practiced in real time. The topic is well described in a number of publications, one such publication being a review paper by Collins, titled “Automatic Rotating Element Ellipsometers: Calibration, Operation and Real-Time Applications”, Rev. Sci. Instrum., 61(8) (1990).
Before proceeding, as it is relevant to the present invention, it is noted that ellipsometer systems generally comprise means for setting a linear or elliptical polarization state, (typically substantially linear).
Continuing, in general, modern practice of ellipsometry typically involves causing a spectroscopic beam of electromagnetic radiation, in a known state of polarization, to interact with a sample system at at least one angle of incidence with respect to a normal to a surface thereof, in a plane of incidence. (Note, a plane of incidence contains both a normal to a surface of an investigated sample system and the locus of said beam of electromagnetic radiation). Changes in the polarization state of said beam of electromagnetic radiation which occur as a result of said interaction with said sample system are indicative of the structure and composition of said sample system. The practice of ellipsometry further involves proposing a mathematical model of the ellipsometer system and the sample system investigated by use thereof, and experimental data is then obtained by application of the ellipsometer system. This is typically followed by application of a square error reducing mathematical regression to the end that parameters in the mathematical model which characterize the sample system are evaluated, such that the obtained experimental data, and values calculated by use of the mathematical model, are essentially the same.
A typical goal in ellipsometry is to obtain, for each wavelength in, and angle of incidence of said beam of electromagnetic radiation caused to interact with a sample system, sample system characterizing PSI and DELTA values, (where PSI is related to a change in a ratio of magnitudes of orthogonal components rp/rs in said beam of electromagnetic radiation, and wherein DELTA is related to a phase shift entered between said orthogonal components rp and rs), caused by interaction with said sample system. The governing equation is:ρ=rp/rs=Tan(Ψ)exp(iΔ)
As alluded to, the practice of ellipsometry requires that a mathematical model be derived and provided for a sample system and for the ellipsometer system being applied. In that light it must be appreciated that an ellipsometer system which is applied to investigate a sample system is, generally, sequentially comprised of:                a. a Source of a beam electromagnetic radiation;        b. a Polarizer element;        c. optionally a compensator element;        d. (additional element(s));        e. a sample system;        f. (additional element(s));        g. optionally a compensator element;        h. an Analyzer element; and        i. a Spectroscopic Detector System.Each of said components b.-i. must be accurately represented by a mathematical model of the ellipsometer system along with a vector which represents a beam of electromagnetic radiation provided from said source of a beam electromagnetic radiation, Identified in a. above)        
Various conventional ellipsometer configurations provide that a Polarizer, Analyzer and/or Compensator(s) can be rotated during data acquisition, and are describe variously as Rotating Polarizer (RPE), Rotating Analyzer (RAE) and Rotating Compensator (RCE) Ellipsometer Systems. It is noted, that nulling ellipsometers also exist in which elements therein are rotatable in use, rather than rotating. Generally, use of a nulling ellipsometer system involves imposing a substantially linear polarization state on a beam of electromagnetic radiation with a linear polarizer, causing the resulting polarized beam of electromagnetic radiation to interact with a sample system, and then adjusting an analyzer to an azimuthal azimuthal angle which effectively cancels out the beam of electromagnetic radiation which proceeds past the sample system. The azimuthal angle of the analyzer at which nulling occurs provides insight to properties of the sample system.
Continuing, in use, data sets can be obtained with an ellipsometer system configured with a sample system present, sequentially for cases where other sample systems are present, and where an ellipsometer system is configured in a straight-through configuration wherein a beam of electromagnetic radiation is caused to pass straight through the ellipsometer system without interacting with a sample system. Simultaneous mathematical regression utilizing multiple data sets can allow calibration of ellipsometers and evaluation of sample system characterizing PSI and DELTA values over a range of wavelengths. The obtaining of numerous data sets with an ellipsometer system configured with, for instance, a sequence of sample systems present and/or wherein a sequential plurality of polarization states are imposed on an electromagnetic beam caused to interact therewith, can allow system calibration of numerous ellipsometer system variables.
It is further noted that it was disclosed in a Co-Pending Parent application Ser. Nos. 12/802,734 and 12/456,791 and 12/802,638, that the present invention is a practical ellipsometer or polarimeter system for application in the range of frequencies between 300 GHz or below. In that light it is to be understood that prior art demonstrates that it is not unknown to propose, or provide a system for, and practice of ellipsometry at Terahertz (THz) frequencies, however, a specific embodiment than makes such possible and which is suitable for general application in universities and industry etc., has not been previously disclosed. To the Applicant's knowledge, there are no commercially available THz ellipsometers or polarimeters available in the market place.
While Synchrotrons have been used to provide THz frequency band electromagnetic radiation in ellipsometers, it is not remotely possible to provide a Synchrotron at every location whereat it is desired to practice THz ellipsometry. The present invention provides combination of many elements, which results in a novel, practical system for general application in the market place.
Before proceeding, it is of benefit to define some terminology. First, a generally accepted range for what constitutes a Terahertz range of frequencies is from 3×1011 (ie. 300 GHz), to 1.3×1012 (ie. 1.3 Thz), Hertz. The Terahertz range is sandwiched between the microwave, (the high end of which has a wavelength of 1 millimeter), and the far-infrared, (the long-wavelength edge of which is 100 micrometers), ranges of wavelengths/frequencies.
Next, it is noted that a number of sources of Terahertz (THz) electromagnetic radiation exit. For instance, a Smith-Purcell cell is a device which directs an energetic beam of electrons very close to a ruled surface of a diffraction grating. The effect on the trajectory of the beam is negligible, but a result is that Cherenkov radiation in the Terahertz frequency range can be created, where the phase velocity of the electromagnetic radiation is altered by the periodic grating. Another source of Terahertz radiation is a Free Electron Laser. In this source a beam of electrons is accelerated to relativistic speed and caused to pass through a periodic transverse magnetic field. The array of magnets is sometimes called an undulator or “wiggler” as it causes the electrons to form a sinusoidal path. The acceleration of the electrons causes release of photons, which is “synchrotron radiation”. Further, the electron motion is in phase with the field of said released electromagnetic radiation, and therefore the fields add coherently. Instabilities in the electron beam resulting from interactions of the oscillations in the undulators lead to emission of electromagnetic radiation, wherein electrons radiate independently. The wavelength of the emitted electromagnetic radiation from the electrons can be adjusted by adjusting the energy of the electron beam and/or magnetic field strength of the undulators, to be in the Terahertz range. Another source, (and preferred in the present invention), of Terahertz frequencies is a Backward Wave Oscillator (BWO), which is a vacuum tube system comprising an electron gun that generates an electron beam and causes it to interact with an electromagnetic wave traveling in a direction opposite to that of ejected electrons such that THz frequency oscillations are sustained by interaction between the propagating traveling wave backwards against the electron beam.
It is also disclosed that numerous detectors exist for monitoring Terahertz range electromagnetic radiation. One example is a Golay cell which operates by converting a temperature change resulting from electromagnetic radiation impinging onto material, into a measurable signal. Generally, when electromagnetic radiation is caused to impinge on a blackened material it heats a gas, (eg. Xenon) in an first chamber of an enclosure, and that causes a distortable reflecting diaphram/film adjacent to said first chamber to change shape. In a second chamber, separated from the first by said diaphram/film an electromagnetic beam is caused to reflect from the film and into a photocell, which in turn converts the received electromagnetic radiation into an electrical signal. A Bolometer is another detector of monitoring Terahertz range electromagnetic radiation, but operates by using the effect of a changing electric resistance caused by electromagnetic radiation impinging onto a blackened metal.
It is also noted that there are Solid State sources and detectors of Terahertz frequency electromagnetic radiation. For instance, an identified reference by Nagashima et al. discloses that THz pulses can be generated by a bow-tie photoconductive radiation antenna excited by a mode-locked Ti-sapphire laser with 80 Fs time width pulses, and a detection antenna can be formed from a dipole-type photoconductive antenna with a 5 micron gap fabricated on thin film LT-GaAs. Further, it is known that a company named AB Millimeter in Paris France, supplies a system that covers the entire range from 8 GHz to 1000 GHz with solid state source and detector devices.
Before disclosing known references, it is noted that computer searching at the PTO Website for Patents and Published Applications containing the words:                (ellipsometer & bolometer); and        (ellipsometer & Golay cell);produced only one hit, that being Published Application US2005/0175507 by Tsukruk. Said 507 reference does contain the words ellipsometry and Golay, but does not describe an ellipsometer system comprising said elements.        
Further, a PTO Website Search for Patents and Published Applications containing the words:                (ellipsometer & backward wave oscillator);        (ellipsometer & Smith-Purcell); and        (ellipsometer & free electron laser);produced only U.S. Pat. No. 5,317,618 to Nakahara et al., which contains the words ellipsometer & free electron laser, but does not describe a combination of said elements.        
A Patent to Wang et al., U.S. Pat. No. 5,914,492 is of interest as it describes free electron lasers used in combination with a Golay cell and Smith-Purcell detectors. However, it does not describe application in ellipsometry or polarimetry.
A Published Application, US2006/0050269 by Brownell describes use of a free electron laser and a Smith-Purcell detector, but not in the context of ellipsometry or polarimetry.
An article titled “Gain of a Smith-Purcell Free Electron Laser”, Andrews et al., Phy. Rev., Vol 7, 070701 (2004), describes use of Smith-Purcell Free Electron Laser.
U.S. Pat. No. 2,985,790 to Kompfner is disclosed as it describes a Backward Wave Oscillator.
U.S. Pat. No. 2,880,355 to Epsztein is disclosed as it describes a Backward Wave Oscillator.
Known References which describe Ellipsometers which operate in the THz frequency range are:                “Terahertz Generalized Meuller-matrix Ellipsometery”, Hofmann et al., Proc. of SPIE, Vol. 6120, pp. 61200D1-61200D10, (2005), describes applying Thz electromagnetic radiation in generalized ellipsometry wherein the source of the Thz electromagnetic radiation is a synchrotron located at BESSY, in Germany.        “Terahertz magneto-optic generalized ellipsometry using synchrotron and blackbody radiation”, Hofmann et al., American Inst. of Physics, 77, 063902-1 through 063902-12, (2006), describes applying Thz electromagnetic radiation in generalized ellipsometry wherein the source of the Thz electromagnetic radiation is a synchrotron and a conventional blackbody. The use of an FTIR source and bolometer is also mentioned.        “Label-free Amplified Bioaffinity Detection Using Terahertz Wave Technology”, Menikh et al., Biosensors and Bioelectronics 20, 658-662 (2004), describes use of an unbiased GaAs crystal THz source of electromagnetic radiation and a ZnTe crystal detector.        Spectroscopy by Pulsed Terahertz Radiation”, Hango et al., Meas. Sci. and Technol., 13 (2002), pp 1727-1738, describes applying 30 GHz-10 THz and describes use of Fourier Transform Spectrometers (FTS) in the Far Infrared (FIR) frequency range with the caution that such an approach is not easily applied below 1 THz. Said reference also describes application of Backward Wave Oscillators (BWO) plus frequency multipliers, with the caution that to cover the range of 30 GHz to 3 THz typically requires many BWO's and frequency multipliers to cover said frequency range. This article favors use of a Femto-sec laser (eg. a mode-locked Ti:sapphire laser or Er-doped fiber laser in combination with a photoconductive antenna made on low-temperature grown GaAs).        “Measurement of Complex Optical Constants of a Highly Doped Si Wafer Using Terahertz Ellipsometry”, Nagashima et al., Applied Phys. Lett. Vol. 79, No. 24 (10 Dec. 2001). This article describes use of a mode-locked Ti:sapphire laser with a bow-tie antenna and GaAs detector antenna).        Published Patent Application No. US2004/0027571 by Luttman mentions using a THz light Source in an ellipsometer system.        “Development of Terahertz Ellipsometry and its Application to Evaluation of Semiconductors”, Nagashima et al., Tech. Meeting on Light Application and Visual Science, IEEE (2002) proposes a Terahertz ellipsometer.        “Terahertz Imaging System Based on a Backward-Wave Oscillator, Dobroiu et al., Applied Optics, Vol. 43, No 30, (20 Oct. 2004) describes use of a Terahertz source to provide electromagnetic radiation.        
A Patent to Herzinger et al. U.S. Pat. No. 6,795,184, describes an “Odd-Bounce” system for rotating a polarization state in an electromagnetic beam. Patents disclosed in the Application leading to U.S. Pat. No. 6,795,184 are:                Patent to Herzinger, U.S. Pat. No. 6,137,618 is disclosed as it describes a Single Brewster Angle Polarizer in the context of multiple reflecting means, and discloses prior art dual Brewster Angle Single Reflective Means Polarizer Systems.        Patent, to Herzinger et al., U.S. Pat. No. 6,084,675 describes an adjustable beam alignment compensator/retarder with application to spectroscopic ellipsometry.        U.S. Pat. No. 6,118,537 to Johs et al. describes a multiple Berek plate optical retarder system.        U.S. Pat. No. 6,141,102 to Johs et al. describes a single triangular shaped optical retarder element.        U.S. Pat. No. 5,946,098 to Johs et al., describes dual tipped wire grid polarizers in combination with various compensator/retarder systems.        U.S. Pat. No. 6,100,981 to Johs et al., describes a dual horizontally oriented triangular shaped optical retarder.        U.S. Pat. No. 6,084,674 to Johs et al., describes a parallelogram shaped optical retarder element.        U.S. Pat. No. 5,963,325 to Johs et al., describes a dual vertically oriented triangular shaped optical retarder element.        U.S. Pat. No. 7,450,231 and U.S. Pat. No. 7,460,230 to Johs et al. are disclosed as they describe deviation angle self compensating compensator systems.        A Patent to Johs et al., U.S. Pat. No. 5,872,630 is disclosed as it describes an ellipsometer system in which an analyzer and polarizer are maintained in a fixed in position during data acquisition, while a compensator is caused to continuously rotate.        A Patent to Thompson et al. U.S. Pat. No. 5,706,212 is also disclosed as it teaches a mathematical regression based double Fourier series ellipsometer calibration procedure for application, primarily, in calibrating ellipsometers system utilized in infrared wavelength range. Bi-refringent, transmissive window-like compensators are described as present in the system thereof, and discussion of correlation of retardations entered by sequentially adjacent elements which do not rotate with respect to one another during data acquisition is described therein.        Further Patents disclosed in the 184 Patent are:                    U.S. Pat. No. 5,757,494; and                            U.S. Pat. No. 5,956,145;                                                to Green et al., in which are taught a method for extending the range of Rotating Analyzer/Polarizer ellipsometer systems to allow measurement of DELTA'S near zero (0.0) and one-hundred-eighty (180) degrees, and the extension of modulator element ellipsometers to PSI'S of forty-five (45) degrees. Said Patents describes the presence of a variable, transmissive, bi-refringent component which is added, and the application thereof during data acquisition to enable the identified capability.        A Patent to He et al., U.S. Pat. No. 5,963,327 is disclosed as it describes an ellipsometer system which enables providing a polarized beam of electromagnetic radiation at an oblique angle-of-incidence to a sample system in a small spot area.        Patents of general interest disclosed in the 184 Patent include:                    Patent to Woollam et al, U.S. Pat. No. 5,373,359, (describes a beam chopper);            Patent to Johs et al. U.S. Pat. No. 5,666,201;            Patent to Green et al., U.S. Pat. No. 5,521,706; and            Patent to Johs et al., U.S. Pat. No. 5,504,582;and are disclosed as they pertain to ellipsometer systems.                        A Patent to Coates et al., U.S. Pat. No. 4,826,321 is disclosed as it describes applying a reflected monochromatic beam of plane polarized electromagnetic radiation at a Brewster angle of incidence to a sample substrate to determine the thickness of a thin film thereupon. This Patent also describes calibration utilizing two sample substrates, which have different depths of surface coating.        Other Patents which describe use of reflected electromagnetic radiation to investigate sample systems are:                    U.S. Pat. No. RE 34,783,                            U.S. Pat. No. 4,373,817,                U.S. Pat. No. 5,045,704                                                to Coates; and                    U.S. Pat. No. 5,452,091                        to Johnson.        A Patent to Bjork et al., U.S. Pat. No. 4,647,207 is disclosed as it describes an ellipsometer system which has provision for sequentially, individually positioning a plurality of reflective polarization state modifiers in a beam of electromagnetic radiation. U.S. Pat. Nos. 4,210,401; 4,332,476 and 4,355,903 are also identified as being cited in the 207 Patent. It is noted that systems as disclosed in these Patents, (particularly in the 476 Patent), which utilize reflection from an element to modify a polarization state can, if such an element is an essential duplicate of an investigated sample and is rotated ninety degrees therefrom, the effect of the polarization state modifying element on the electromagnetic beam effect is extinguished by the sample.        A Patent to Mansuripur et al., U.S. Pat. No. 4,838,695 is disclosed as it describes an apparatus for measuring reflectivity.        Patents to Rosencwaig et al., U.S. Pat. Nos. 4,750,822 and 5,596,406 are also identified as they describe systems which impinge electromagnetic beams onto sample systems at oblique angles of incidence. The 406 Patent provides for use of multiple wavelengths and multiple angles of incidence. For similar reasons U.S. Pat. No. 5,042,951 to Gold et al. is also disclosed.        In addition to the identified Patents, certain Scientific papers were also disclosed in the 184 Patent are:        A paper by Johs, titled “Regression Calibration Method for Rotating Element Ellipsometers”, Thin Solid Films, 234 (1993) is also disclosed as it describes a mathematical regression based approach to calibrating ellipsometer systems.        
An additional relevant Patent is U.S. Pat. No. 6,268,917 to Johs. This Patent describes a combined polychromatic electromagnetic radiation beam source comprising beam combiners.
It is also disclosed that the J. A. Woollman Co., Inc. has marketed an IR range Ellipsometer, called the IR-VASE (Reg. TM), for many years. Said instrument provides capability from 10 THz to 150 THz and is a Variable Angle, Rotating Compensator system utilizing a Bomen FTIR Spectrometer. Further, it comprises an FTIR Source, and an Odd-Bounce image rotating system for rotating a polarization state imposed by a wire-grid polarizer. It is noted that as marketed, this system has never provided the capability to reach down to 1 THz, which capability was achieved via research in developing the present invention.
Additional references which describe ellipsometry practiced in the THz range are:                “THz Ellipsometry in Theory and Experiment”, Dietz et al. 33rd International Conference on Infrared and Millimeter Waves and 16th International Conference on Terahertz Electronics, IRMMW-THz (2008) describes an experimental ellipsometer for use in the THz frequency range;        “Study Terahertz Ellipsometry Setups for Measuring Metals and Dielectrics Using Free Electron Laser Light Source”, Rudych, 31st International Conference on Infrared and Millimeter Waves and 14th International Conference on Terahertz Electronics, IRMMW-THz (2006) describes use of a free electron laser to provide THz frequencies;        “Spectral THz Ellipsometer for the Unambiguous Determination of all Stokess Parameters”, Holldack et al., 30th International Conference on Infrared and Millimeter Waves and 13th International Conference on Terahertz Electronics, IRMMW-THz (2006) describes a concept for determining all Stokes Parameters;        “Terahertz Magneto-Optic Generalized Ellipsometry Using Synchrotron and Blackbody Radiation”, Esquinazi et al., Sci. Instrum., Vol. 7, No. 6 (2006) describes use of synchrotron generated electromagnetic radiation in magneto-optic generalized ellipsometry;        “Terahertz Generalized Mueller-Matrix Ellisometry”, Esquinazi et al. Proc. Int. Soc. Opt. Eng., Vol. 6120, (2006) describes synchrotron generated electromagnetic radiation in generalized Mueller Matrix ellipsometry        “THz Time-Domain Magneto-Optic Ellipsometry in Reflection Geometry”, Kuwata-Gonokami et al., Trends Opt. Photonics Series, Vol. 97, (2004) describes determining a dielectric tensor using THz frequencies in magneto-optic optical measurements;        “Terahertz Polarimetry”, Gallot et al., Conf. Lasers Electro-Optics, CLEO, Vol. 3 (2005) describes determining the polarization state of a THz wave over a wide range of frequencies;        “Evaluation of Complex Optical Constants of Semiconductor Wafers using Terahertz Ellipsometry”, Hangyo et al., Trends Opt. Photonics Series, Vol. 88, (2003) describes combined terahertz ellipsometry with time domain spectroscopy.        
Additional references which describe sources of Terahertz frequency range electromagnetism are:                “Improved Performance of Hybrid Electronic Terahertz Generators”, Hurlbut et al., 33rd International Conference on Infrared and Millimeter Waves and Terahertz Waves, IRMMW-THz (2008), describes combining BWO's with frequency multipliers;        “Terahertz Wave Generation in Orientation-Patterned GaAs Using Resonantly Enhanced Schemes”, Vodopyanov et al., SPIE-Intl. Soc. for Opt. Eng. USA, Vol. 6455, (2007), describes application of Zincblende semiconductors (GaAs, GaP) to produce THz frequencies;        “Terahertz BWO Spectroscopy of Conductors and Superconductors”, Gorshunov et al., Quantum Electronics, Vol. 37, No. 10 (October 2007), describes methods for directly measuring dielectric response spectra of dielectrics, conductors and superconductors using BWO generated spectrometers;        “Portable THz Spectrometers”, Kozlov et al., 31st International Conference on Infrared and Millimeter Waves and 14th International Conference on Terahertz Electronics, IRMMW-THz (2007), describes a portable THz spectrometer which operates in the frequency range of 0.1-1 THz;        “Terahertz Time-Domain Spectrsocopy”, Nishizawa et al., Terahertz Optoelectronics, Topics Appl. Phys. 97, 203-271 (2005).        U.S. Pat. No. 7,339,718 to Vodopanov et al., Issued Apr. 3, 2008 describes a method for generating THz radiation comprising illuminating a semiconductor with an optical pulse train.        U.S. Pat. No. 6,819,423 to Stehle et al., Issued Nov. 16, 2004 and U.S. Pat. No. 5,317,618 Issued Jan. 25, 2005 are also identified as they mention application of THz frequencies in an ellipsometer system.        
It is noted that the Search Report for a co-pending PCT Application, PCT/US09/05346, was recently received. It identified the following references: U.S. Pat. Nos. 6,795,184; 7,274,450 and 6,798,511; and Published Applications Nos. US2004/0228371; US2007/0252992; US2006/0289761; US2007/0278407; US2007/0097373. Also identified were: a Ph.D. dissertation by Duerr, Erik Kurt, titled “Distributed Photomixers”, Mass. Inst. Tech., September 2002; and article titled “Hole Diffusion Profile in a P−P+ Slicon Homoiunction Determined by Terahertz and Midinfrared Spectroscopic Ellipsometry”, Hofmann et al., App. Phys. Lett., 95 032102 (2009).
The identified references, application Ser. No. 12/456,791, Provisional Application Ser. No. 61/208,735 and Ser. No. 61/281,905, are all incorporated by reference into this Specification.
Further, in view of Examine Action leading to Allowance of Parent application Ser. No. 13/506,848 it is noted that the Published Application by Nagashima et al. No. 2003/0016358 specifically calls for application of a LASER, (ie. a Single Wavelength), Source within a range of 0.2 to 0.8 THZ. In view thereof, Claims herein are presented herein which specifically avoid the Nagashima et al. No. 2003/0016358 limited THZ range and require a non-laser Source in the Present Invention.
In addition, it is noted that the cited Nagashima et al. No. 2003/0016358 reference describes a time-domain polarization analyzing apparatus. It is completely different from an Ellipsometer/polarimeter. It is also emphasised that the present invention provides a continuous wave Source, whereas the 358 reference applies a Laser Source. Further, the 358 reference splits the pulses generated and directs them along different pathways, then recombine them to provide interference. It then describes calculating sample characteristics much differently than does the present invention. Further, although not particularly important, it was noted that the element (50) in the 358 reference is a computer, not the detector, which is element (7). And, it was noted to me that the present invention Polarizer does not change direction of a beam, it simply sets a polarization state therein. If required to further distinguish the present invention, the relevant Polarizers in the present invention could be designated as not changing the direction of the beam as shown in FIG. 8a of the Original Specification.
Finally, nothing in the 358 reference remotely suggests using other than a Laser Source, or doing away with the use of Pulsing or Interference, as the invention therein would simply not work if any of those things were done. Therefore, nothing in the 385 reference remotely leads one to the Present Invention. Simply because some remotely similar elements are present in the 385 system and the Present Invention System, (which are differently applied to arrive at different results in different ways), does not remotely obviate the Present Invention. There are no instructions in any cited prior art that would guide one skilled in the art to begin with the 385 invention and arrive at the Present Invention!
Even in view of relevant prior art, there remains need for an ellipsometer or polarimeter system for application in the Terahertz region, preferably in combination with a convenient approach to providing linearly polarized beams of electromagnetic radiation in which the azimuthal angle of the linear polarization can be controlled.